1. Field of the Invention
The present invention relates to microphones, and more particularly to a microphone for the transmission and conversion of low frequency body sounds, the microphone being of the type having a shallow housing whose open side carries a peripherally clamped membrane interacting with a Seignette salt crystal wafer arranged inside the housing, through the intermediate of a transmitter body.
2. Description of the Prior Art
Microphones of the above-mentioned type are used with great success for the transmission of body sounds in conjunction with blood pressure measuring devices, the microphone being normally arranged on the inside of the measuring sleeve of the blood pressure measuring device, so that it is placed in contact with the limb on which the blood pressure is measured. In certain situations, however, such a body sound microphone is not arranged directly on the measuring sleeve, but mounted on a microphone adapter attached to the measuring sleeve, and in other, rare, cases, the microphone may be mounted on a separate implement which is fastened to the limb in question independently of the measuring sleeve of the blood pressure measuring device. Microphones for the transmission of body sounds serve for the determination of blood pressure characteristics in the person which is being examined, the microphone being capable of transmitting the so-called Korotkoff noises which are generated by the blood circulation downstream of the measuring sleeve, when the air pressure in the sleeve reaches a certain range of excess pressure. The microphone transforms these sounds into electronic signals which are then transmitted to the blood pressure measuring device and which, following appropriate amplification, can be used to affect certain controls on the measuring device, by operating certain switching functions, depending upon the design of the blood pressure measuring device.
The principal component of such a body sound microphone is a Seignette salt crystal wafer of square outline which is clamped elastically on three of its corners by means of suitable clamping blocks attached to the microphone housing in such an orientation that the fourth corner of the crystal wafer is positioned at a place that coincides approximately with the geometric center axis of the microphone housing and of the attached membrane. Between the lower side of this membrane and the upper side of the crystal wafer is normally arranged a transmitter body of solid plastic material which transmits the vibrations induced in the membrane by the body sounds to the free corner of the crystal wafer. The latter thus acts as a crystalline flexing member. The deflection of the crystal wafer creates piezoelectric voltage variations which are picked up by leads connected to the two sides of the crystal wafer, which leads transmit the signals to a signal amplifier.
Seignette salt crystal wafers have proved to have the highest presently obtainable signal transmission efficiency. They also have a serious shortcoming, however, in that they are very sensitive to mechanical stress. Because their signal transmitting efficiency is inversely related to the thickness of the wafer, the risk of fracture of such a wafer increases with increasing transmission quality. For this reason, Seignette salt crystal wafers are not presently used in the manufacture of body sound microphones, inspite of their excellent piecoelectric characteristics. In their place are presently used ceramic piezoelectric receivers of approximately the same shape, in approximately the same mounting arrangement.
Piezoelectric ceramic wafers have a much greater resistance against mechanical stress. Their resistance is adequate for their incorporation in blood pressure measuring devices, without the need for additional safety precautions. However, the comparatively reduced risk of fracture of piezoelectric ceramic wafers is accompanied by a much reduced efficiency of transmission, as compared to the Seignette salt crystal wafer. The former type of wafer therefore necessitates a much greater signal amplification. This need, in turn, brings with it the problem that the natural capacitance of the connection to the amplifier becomes an influential part of the transmitted signal. When such a receiver element is interchanged, or when the connection to the amplifier is replaced, there results a change in the impedance which makes it necessary in each case to rebalance the input amplifier. In order to equalize the changes in capacitance among different receivers, known instruments are equipped with a preamplifier in the receiver housing, the preamplifier being electrically interposed between the ceramic wafer and the near end of the connecting leads to the amplifier. This preamplifier does not equalize capacitance changes in the connecting leads, however, so that the latter have to be separately balanced. In a situation where a blood pressure measuring device is equipped with several measuring sleeves and associated receivers, and where the measuring sleeves are used interchangeably, it becomes necessary to rebalance the input amplifier in the instrument, whenever a different measuring sleeve is used, because there exist unavoidable manufacturing tolerances between the various preamplifiers in the different microphones.